1. Field of the Invention
The present invention relates to gyroscopes, and in particular to improved resonator microgyroscopes and their manufacture.
2. Description of the Related Art
Gyroscopes are used to determine direction based upon the sensed inertial reaction of a moving mass. In various forms they are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to determine the orientation of a free object.
Older conventional gyroscopes were very heavy mechanisms, employing relatively large spinning masses by current standards. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as vibratory gyroscopes.
Spacecraft often depend on inertial rate sensing gyroscopic systems to supplement attitude control. Typical systems employ conventional spinning mass gyroscopes or conventionally-machined hemispherical resonator gyroscopes to provide the high pointing stability required for spacecraft payloads operating high above the earth. However, both of these types of gyroscopes are expensive, large and heavy.
Some prior symmetric vibratory gyroscopes have been produced, however, their vibratory momentum is transfered directly to their baseplates or packages. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation. In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and vibrating ring gyroscope, which can have desirable isolation and vibrational symmetry attributes, are not suitable for thin planar silicon implementation with sensitive electrostatic sensors and actuators that take advantage of the large planar areas of the device
The scale of previous silicon microgyroscopes (e.g., U.S. Pat. No. 5,894,090) has not been optimized for navigation grade performance resulting in higher noise and drift than desired. This problem stems from use of thin eptiaxially grown silicon flexures to define critical vibration frequencies that are limited to 0.1% thickness accuracy and limit device sizes to a few millimeters. The former results in high drift due to vibrational asymmetry or unbalance and the latter results in high rate noise due to lower mass increasing thermal mechanical noise and lower area increasing capacitance sensor electronics noise. Scaling up of non-isolated silicon microgyros is also problematic because external energy losses will increase with no improvement in resonator Q and no reduction in case-sensitive drift. An isolated, cm scale resonator with many orders of magnitude in 3D manufacturing precision is required for navigation grade performance. Conventionally machined navigation grade resonators such as in hemispherical or shell gyros have the optimum scale, e.g. 30 mm and 3D manufacturing precision and hence desirable drift and noise performance, however are expensive and slow to make. Conventional laser trimming of mechanical resonators can further improve manufacturing precision to some degree, however it is not suitable for microgyros with narrow mechanical gaps and has limited resolution necessitating larger electrostatic bias adjustments in the final tuning process.
There is a need in the art for small microgyros with greatly improved performance for navigation and spacecraft payload pointing. There is also a need for such gyros to be cheaper and more easily manufactured with greater 3D mechanical precision. Finally, there is a need for such gyros to have desirable isolation and vibrational symmetry attributes while being compatible with planar silicon manufacturing. The present invention satisfies all these needs.